The invention relates to a flat gasket with a gasket plate which has at least one hole for the passage of a threaded shaft, in particular, the threaded shaft of a screw and is formed in the region of this hole by a sheet-metal layer with a relatively slight sheet-metal thickness.
Above all in the case of combustion engines, flat gaskets are used ever more frequently, for example, as exhaust manifold gaskets and cylinder head gaskets and their gasket plate consists essentially of one or more sheet-metal layers which is or are, where applicable, coated completely or partially with a, for example, elastomeric mass, wherein in the case of multiple-layer gasket plates one sheet-metal layer can form a carrier plate which is provided on one or both sides with a support formed from an additional sheet-metal layer and not congruent with the carrier plate so that a screw hole can be formed only in one of these sheet-metal layers.
Screws or threaded bolts, which are pushed through the screw holes of the gaskets, serve for the assembly of such flat gaskets, wherein during the assembly or during the transport of the gaskets to the place of assembly the gaskets can hang on the screws (in this case, the plane of the gasket plate extends at least approximately vertically); there is then, above all, in the case of a sheet-metal layer with a correspondingly slight sheet-metal thickness the risk of the edge region of this sheet-metal layer, which borders on a screw hole, slipping into a thread of the screw or the threaded bolt and, consequently, of the screw or the threaded bolt no longer being located centrally in the circular screw hole. When a gasket is to be positioned correctly and exactly on the engine by means of the screws or threaded bolts, the result of such an engagement of the sheet-metal layer in a thread is that the gasket, in certain circumstances, is inadvertently not assembled in the required position relative to structural parts of the engine which are to be sealed relative to one another, which can, for example, lead to leakages because modern engine constructions normally require a very exact positioning of sealing elements, such as, for example, beads, of such gaskets.
To avoid this problem in the case of a flat gasket with a gasket plate which has at least one hole for the passage of a threaded shaft and is formed in the region of this hole by a sheet-metal layer, the sheet-metal thickness of which would make an engagement of an edge region bordering on the hole in a thread of the threaded shaft possible, it is suggested in accordance with the invention to design the gasket such that for the centering of the threaded shaft in the hole the sheet-metal layer has in its edge region bordering on the hole at least one centering element which is formed by the sheet-metal layer and bent out of its plane, prevents any engagement of the sheet-metal layer in a thread and during the tightening of the screw can be bent back at least approximately into the plane of the sheet-metal layer without any great expenditure of force, namely such that as a result the centering element is pressed at least almost flat and cannot lead, for example, on account of any formation of folds or the like to any undesired, appreciable thickening of the sheet-metal layer in the region of the said hole such as could be the case when a sheet-metal tongue projecting from the edge of the hole is used as centering element and this has been bent out of the plane of the sheet-metal layer such that it forms with this plane an angle of, for example, 90xc2x0 and abuts against the circumference of the threaded shaft.
When, as is often the case, flat gaskets with a gasket plate oriented approximately vertically are mounted in a suspended manner on the said screws or threaded shafts, namely such that a quite specific outer edge region of the gasket plate is located at the top, a single centering element is sufficient per screw hole and this is arranged such that it is supported from above on the outer circumference of the threaded shaft; when these preconditions need not, however, always be present, it is recommended that the gasket be designed such that the sheet-metal layer has several centering elements which are arranged, in particular, at a distance from one another (around the screw hole).
So that the centering element can be bent back as easily as possible into the plane of the sheet-metal layer, it is advantageous when it extends only over the circumferential angle of the screw hole required for the centering of the screwxe2x80x94if the extension of the centering element in circumferential direction of the screw hole is too small, it is not ensured for every design of the centering element that this cannot engage in a thread.
In principle, the centering element can have the most varied of geometrical shapes; when seen from the center of the screw hole towards the rim of the edge region of the sheet-metal layer bordering on the screw hole, the centering element can have, for example, the following shapes: the shape of an approximately semicircular bead, an S lying on its side or a V or, however, even the shape of a diagonally drawn-out Z, wherein the middle arm of the Z extends approximately transversely to the direction of the thread pitch; the centering element can, however, also be formed in that the edge region of the sheet-metal layer surrounding the screw hole is slotted in an approximately radial direction and then one of the two regions bordering on the slot or both the regions of the sheet-metal layer bordering on the slot is or are bent up out of the plane of the sheet-metal layer in the direction of the axis of the hole (in the case where both regions are bent up, these can be bent out of the plane of the sheet-metal layer in opposite directions), and, finally, it is also conceivable to provide two slots arranged at a slight distance from one another and extending approximately radially in relation to the axis of the hole and to fold the sheet-metal tongue thereby resulting and located between the two slots in relation to the plane of the sheet-metal layer, i.e. tilt the tongue somewhat about the central axis.
In all the cases, the centering element can not only be returned to its shape easily but it also allows, and also in the case of several centering elements per screw hole, the gaskets to be fitted onto the threaded shafts easily without the edge of the hole hooking on the ridges of the threads which has previously made the assembly procedure more difficult.
Embodiments are particularly preferred, with which the centering element is connected to and merges into the actual sheet-metal layer at its edges extending transversely to the edge of the screw hole, since not only problems with the packaging of flat gaskets can be avoided with embodiments of this type but also any risk of injury during the handling of inventive gaskets.
With embodiments of this type, the centering element could be punched free at its edge facing away from the screw hole; such inventive gaskets are, however, preferred, with which the centering element is also connected to and merges into the actual sheet-metal layer at its edge facing away from the screw hole.
It is of advantage not only with respect to the production but also the functioning of the centering element and the ease with which this can be bent back when this has the shape of a burl stamped out of the sheet-metal layer and open towards the center of the hole.
The centering element may be bent back into the plane of the actual sheet-metal layer during the assembly of the gasket the easiest and certainly pressed flat without any formation of folds when the edge region of the centering element facing the center of the screw hole extends parallel to the actual sheet-metal layer.